Simple valid BST
Input
TreeNode(2, TreeNode(1), TreeNode(3))
Output
True
Left child 1 < 2 and right child 3 > 2; both subtrees are valid.
Full lesson preview
Validate a BST
Check whether a binary tree is a valid Binary Search Tree (BST).
Python practice14 minTrees & Binary TreesIntermediateLast updated March 29, 2026
Problem statement
Given the root of a binary tree, determine if it is a valid Binary Search Tree (BST). A valid BST is defined as follows: the left subtree of a node contains only nodes with keys strictly less than the node's key, the right subtree contains only nodes with keys strictly greater than the node's key, and both left and right subtrees must also be binary search trees. Return True if the tree is a valid BST, otherwise return False.
Task
Implement an algorithm to verify that a binary tree satisfies BST ordering: left < node < right for every node.
Examples
Input format
A single binary tree root (TreeNode or None).
Output format
Boolean value: True if the tree is a valid BST, otherwise False.
Constraints
The number of nodes is in the range [0, 10^4]. Node values are integers and may be negative. Use O(n) time and O(h) auxiliary space where h is the tree height.
Samples
Sample 1
Input
TreeNode(5, TreeNode(1), TreeNode(4, TreeNode(3), TreeNode(6)))
Output
False
The right subtree has a node with value 3 which is not greater than 5.